linux: make chrome's ozone platform selection code reusable

Ozone platform 'auto' selection (X11 vs Wayland) logic is currently in
chrome layer, that was a design decision to keep it out of Ozone scope,
as it is a (temporary) Linux desktop specific feature which somewhat
conflicts with the original and simple ozone platform selection logic,
which basically relies in compile-time flags.

This moves that logic down to //ui/linux instead, making it reusable for
embedders other than Chrome.

Moreover, this includes a few more additional fixes/improvements found
along the way:

- Fix the XDG_SESSION_TYPE= WAYLAND_DISPLAY=
  XDG_RUNTIME_DIR=/run/user/xxx case, which was broken
- Remove ScopedBlockingCall, which is no longer needed (after
  crrev.com/1275306.

Bug: 330210407
Change-Id: Id79b03e09bd3d4603258ff9429d093bb8fcda783
Test: Manually by running chrome with different ozone-platform-hint values.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5379474
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1281209}